This invention relates to a process for improving ethanol yield and operating efficiency in an ethanol fermentation process. The process includes several steps which can be operated independently or in combination to optimize the ethanol fermentation process. Some of the steps include, but are not limited to, pre-fermentation fatty acid destruction, surfactant addition to enhance enzyme activity, thin stillage water separation and recycling, and use of anaerobic digestion of solids.
Ethanol fermentation is the biological process by which sugars such as glucose, fructose, and sucrose, are converted into ethanol and carbon dioxide. Yeasts carry out ethanol fermentation on sugars in the absence of oxygen. Because the process does not require oxygen, ethanol fermentation is classified as anaerobic. Ethanol fermentation is responsible for the production of ethanol in alcoholic beverages and for much of the production of ethanol for use as fuel.
The three basic types of fermentable plant-based feedstock are saccharine (sugar containing) materials, starchy materials, and cellulose materials. Saccharine materials contain simple, directly fermentable six and twelve carbon sugar molecules such as glucose, fructose, and maltose. Such materials include sugar cane, sugar beets, fruit, citrus molasses, cane sorghum, whey and skim milk. Starchy materials contain more complex carbohydrates such as starch and inulin that can be broken down into the simpler six and twelve carbon sugars by hydrolysis with acid or by the action of enzymes in a process called malting. Such materials include corn, grain sorghum, barley, wheat, rice, potatoes, sweet potatoes, and so on. Cellulose materials, such as wood, wood waste, paper, straw, corn stalks, corn cobs, cotton, etc., contain material that can be hydrolyzed with acid, enzymes or otherwise converted into fermentable sugars called glucose.
Manufacturing ethanol from saccharine feedstocks generally requires: extraction or crushing to make the sugars available to the yeast enzymes during fermentation; dilution, which is only required with certain materials; fermentation; and distillation. Starchy materials require the steps of: milling to free the starchy material from, for example, grain kernels; dilution; cooking to dissolve and “gelatinize” the starch; and conversion of the starch to fermentable sugars by malting, enzymes, or acid hydrolysis in addition to the steps of fermentation and distillation. Cellulose materials are similar to starchy materials in that they must be converted to fermentable carbohydrates prior to fermentation.
In the United States, the main feedstock for the production of ethanol is currently corn. Approximately 2.8 gallons of ethanol are produced from one bushel of corn (0.42 liter per kilogram). While much of the corn turns into ethanol, some of the corn also yields by-products such as DDGS (distillers dried grains with solubles) that can be used to fulfill a portion of the diet of livestock. A bushel of corn produces about 18 pounds of DDGS. Although most of the fermentation plants have been built in corn-producing regions, other feedstocks may be used, including by not limited to sorghum and pearl millet.
FIG. 1A is a block diagram of a typical ethanol plant utilizing a dry milling process 10. It will be understood that this process may be used with a variety of feedstocks, including the feedstocks mentioned above. FIG. 1B is a schematic representation of process equipment to perform the ethanol fermentation process of FIG. 1A. The major steps are outlined below.
The feedstock milling 12 may be performed using hammer mills or other milling means known in the art, which grind it into a fine powder called meal 14. The feedstock may be corn, barley, wheat, or other feedstock mentioned above. The meal 14 is prepared into a mash suitable for fermentation. Mash preparation 16 may include mixing the meal with water to form the mash. Enzymes 18 are added to convert starch into fermentable sugars, a process called saccharification. Ammonia 20 may be added for pH control and as a nutrient to the yeast. The prepared mash 22 is processed in a high-temperature cooker to reduce bacteria levels ahead of fermentation. The mash is cooled and transferred to one or more fermenters for fermentation 24. Yeast is added to the mash to ferment the sugars to ethanol and carbon dioxide 26. Carbon dioxide 26 is given off in great quantities during fermentation. Many ethanol plants collect the carbon dioxide, clean it of any residual alcohol, compress it and sell it for use to carbonate beverages or in the flash freezing of meat. Using a continuous process, the fermenting mash will be allowed to flow, or cascade, through several fermenters until the mash is fully fermented and then leaves the final tank. In a batch fermentation process, the mash stays in one fermenter for about 48 hours before the distillation process is started.
After fermentation, the resulting “beer” 28 is transferred to distillation columns where distillation 30 separates the ethanol 32 from the remaining “stillage” 34. The stillage 34 contains non-fermentable solids from the feedstock and the yeast cells. The ethanol 32 undergoes dehydration 36 in a molecular sieve system to form approximately 200 proof (anhydrous) ethanol 38.
The stillage 34, also referred to as whole stillage, undergoes centrifugation 40 to separate wet distillers grain 42 from thin stillage 44. The wet distillers grain 42 includes the course grain and is typically dried to form dried distillers grains. Distillers grains, wet and dried, are high in protein and other nutrients and are a highly valued livestock feed ingredient. The thin stillage 44 includes solubles, and some ethanol plants use evaporation 46 to remove water from the thin stillage 44 to create a “syrup” containing Condensed Distillers Solubles (CDS) that can be a separate production product. The coarse grain and the syrup may be combined and dried together in a dryer 50 to produce dried distillers grains with solubles (DDGS) 52, a high quality, nutritious livestock feed.
A water condensate 54 from the evaporators 46 may be recovered and recycled in the process.
Many fermentable plant-based feedstocks contain oils that consist principally of triglycerides (also known as triglycerols). Triglycerides are fatty acid esters of glycerol. The fatty acids have various compositions depending on the plant source. For example, corn oil contains about 99% triacylglycerides, which include approximately 59% polyunsaturated fatty acid, 24% monounsaturated fatty acid, and 13% saturated fatty acid. Some of the more common fatty acids present in corn oil include palmitic, stearic, oleic, and linoleic acid.
While the use of stillage in animal feed applications is desirable, stillage contains high quantities of fatty acids that limit its usefulness as an animal feed. Presently, stillage can only represent a small fraction of the diet of livestock and poultry. For example, dairy cows can only consume from about 4 to 8 pounds of stillage per day per cow. It would be a significant advancement in the art to reduce the quantity of fatty acids present in stillage to render it more usable as an animal feed and in other post-fermentation applications.
Feedstock materials used in ethanol fermentation processes often contain complex carbohydrates that are not fermentable under current ethanol fermentation processes. It would be an advancement in the art to provide means for fermenting more carbohydrates present in feedstock materials.